The goal of this research is to study the expression and release of mouse mammary tumor virus proteins from cells of late-occurring murine mammary tumors in an effort to quantitatively and qualitatively examine these viral proteins as diagnostic markers for mammary tumors. Previous work has indicated that the major virion glycoprotein of the mouse mammary tumor virus, gp52, is a useful systemic marker for the presence of early murine mammary tumors in mice carrying MMTV-S. Plasma levels of this protein have been shown to be a means of monitoring murine mammary tumor status in mice receiving chemotherapeutic protocols. Preliminary work has indicated that gp52 is a more abundant plasma marker for the presence of tumor than MMTV's nucleoid protein p27. Determinants of gp52 also have been detected in plasma of mice bearing x-ray and urethane-induced mammary tumors as well as certain nonmammary tumors. Attempts to analyze the molecular form of gp52 determinants released into culture fluids have indicated that mature gp52 and a larger molecular weight form, similar to the precursor, gPr75, are detected. An isotopic staphylococcal protein A test has also been developed to compare the abundance of different viral proteins on the surface of mammary tumor cells. In addition, this test has been utilized in preliminary studies with monoclonal antibodies against gp52 determinants to characterize the presence and abundance of both type-specific and shared MMTV gp52 determinants on the surface of C3H and GR mammary tumor cells.